In this specification we will refer to airborne surveys, and more particularly to gravity gradient surveys. However the techniques we describe are not limited to these types of survey and may be applied to other potential field surveys including, but not limited to, gravity surveys, magnetic field surveys such as magnetotelluric surveys, electromagnetic surveys and the like.
A potential field survey is performed by measuring potential field data which, for a gravity survey, may comprise one or more of gravimeter data (measuring gravity field) or gravity gradiometer data (measuring gravity field gradient), vector magnetometer data, true magnetic gradiometer data, and other types of data well-known to those skilled in the art. A common aim of a geophysical potential field survey is to search for signatures which potentially indicate valuable mineral deposits.
Conventionally airborne potential field surveys such as gravity surveys are flown on a grid pattern. The grid is defined by orthogonal sets of parallel lines (flight paths) on a two-dimensional surface which is draped over the underlying terrain. However the draped surface is constrained by the closest the aircraft is permitted to fly to the ground and the maximum rate of climb/descent of the aircraft. Some improved techniques for airborne potential field surveys, which facilitate the collection of data from close to the ground, are described in the applicant's co-pending PCT patent application “Gravity Survey Data Processing”, PCT/GB2006/050211, hereby incorporated by reference in its entirety.
After the potential field data has been collected but prior to interpreting the data a terrain correction is generally applied, compensating for surface height variations. Surface data may be purchased in the form of digital terrain elevation data or determined from (D)GPS ((Differential) Global Position System) and/or airborne techniques such as LIDAR (Laser Imaging Detection and Ranging) and SAR (synthetic aperture radar). Aircraft acceleration, attitude, angular rate and angular acceleration data may also be used to correct the output data of the potential field instrumentation. We describe some improved techniques for terrain correction in geophysical surveys in our co-pending UK patent application “Terrain Correction Systems”, no. 0601482.3, filed 25 Jan. 2006, also hereby incorporated by reference in its entirety.
Another technique, described in WO 03/032015, corrects measurements from geophysical instruments in real time at source from other navigation and mapping instruments carried by the aircraft. However in practice this type of “on-line” correction suffers from a number of drawbacks.
Thus there remains a need for improved data processing techniques. One problem, for example, arises where the terrain changes rapidly so that spatial aliasing can arise, more particularly where the terrain has peaks or other variations on a length scale which is less than the distance between the survey (flight) lines.